Method for hot briquetting calcium phosphate ore



July 28, 1970 T. E. KASS T AL 5 3 METHOD FOR HOT BRIQUETTING CALCIUM PHOSPHATE ORE Filed May 51, 1968 INVENTORS THEODORE E. KASS ROBERT J. GLEASON ATTORNEY United States Patent O US. Cl. 264-125 3 Claims ABSTRACT OF THE DISCLOSURE A mixture of washed calcium phosphate ore and phosphate ore flotation concentrate, reduced to 35 to 150 mesh size, is heated to 1400-1900 F. to calcine the mixture and while still within this temperature range is compressed under highpressure into briquettes in which the silica sand contained in the ore is bonded together with essentially fused phosphate ore.

BACKGROUND OF THE INVENTION The handling of phosphate ore for reduction in an electric furnace presupposes a certain physical condition of the ore and a chemical balance of ingredients charged to the furnace. That is, the calcium phosphate ore must be melted in the presence of snflicient silica to form a calcium silicate flux of a sufficiently fluid nature so that it can be tapped out of the furnace. Carbon in the furnace must be present in a lump form for reaction with the ore. Similarly, the ore must be charged to the furnace in a lump, or brick, or briquette form, because to do otherwise with finely divided ore, for example, would be to induce the formation of crusts on the molten furnace contents, thereby, to smother gas being released, to create the possibility of explosions, and other operating difficulties. These are known techniques. Generally phosphate ores obtained in the western part of the United States will contain silica and come through a mining operation in a form which may be used as such for electric furnace feed, subject only to agglomerating and calcining. Western ores are of sufficiently high grade, in places (averaging near 24% P that the contained clays which act as a binder in agglomeration need not be removed in a beneficiation process.

Eastern ores, particularly, North Carolina and Florida ores, are of substantially lower grade, 13% to 15% P 0 so that not only the clays must be removed by a washing process, but also some of the quartzite sands must be removed by froth flotation in the case of preparing the ore for electric furnace feed.

Ores obtained in the eastern part of the United States, particularly the types occurring in North Carolina and Florida, occur with quartzite sands and clay and come through the mining operation in a low grade requiring beneficiation. Simple washing removes the clay and leaves a granular phosphate-ore-silica mix. The material is then generally subjected to froth flotation where most of the silica is removed. Florida ore would typically combine clay, sand and mineral in roughly equal proportions. Western ore is over half mineral, with the remaining material mostly clay. The clay in the western ore facilitates agglomeration for electric furnace use.

Hence, since the feed to an electric furnace for the production of elemental phosphorus must necessarily be of a size range and particle strength adequate to resist physical degradation as a result of the mechanical handling, we have devised a method of briquetting as a satisfactory means of agglomerating a Washed enriched ore which occurs as a granular material for use in the electric ice furnace and have also devised a novel form of phosphate ore briquette suitable for furnace feed.

It is, accordinngly, a basic object of the invention to provide a method of briquetting a phosphate feed for a furnace and to provide a phosphate feed briquette suitable for use in the electric furnace which has the advantage of adequate strength to resist attrition in handling, and will have the further advantage that its chemical composition is balanced for the reduction reaction which is carried out in the electric furnace so that additional silica is not required.

DETAILED DESCRIPTION OF THE INVENTION In accordance with the invention, eastern ores, e.g. ores from Florida and North Carolina phosphate deposits, are subjected to a beneficiation process, wherein the included or occluded clays, those finer than, for example, 200 mesh are removed by washing. The material remaining after washing of a typical North Carolina or Florida ore will contain quartzite sand which can be separated from the ore in a froth flotation process and a very refined ore can be prepared in a double froth flotation. Thus, in the beneficiation (washing) the clay binder of the ore is removed. The silica or quartzite sand occurring with the ore is just about suflicient for the furnace feed when the washed ore is enriched with concentrate from a flotation operation.

A typical North Carolina phosphate ore analysis would be about as follows:

TYPICAL CHEMICAL ANALYSES, NORTH CAROLINA In accordance with this invention, therefore, a close approximation to an ideal agglomerated furnace feed is prepared using beneficiated North Carolina or Florida phosphate type ores by developing a blend of the flotation concentrate and washed ore containing quartzite sand and, thereafter, briquetting the mixture. The dry mix appropriately balanced in its chemical constitution for use as a furnace feed is dried, calcined and passed through a hot briquetting machine and, in the briquetting rocess, the combination of heat and pressure causes part of the phosphate material to become plastic or semi-plastic at temperatures considerably below their normal melting or softening temperatures, with the result that we have been able to make, through this manipulation of washed ore and concentrate and our hot briquetting operation, briquettes of phosphate rock containing substantial amounts of quartzite (high silica sand), despite the fact that quartzite itself does not become plastic or approximately semi-plastic at any temperature within ordinary operating limits of hot briquetting apparatus. Also the abrasiveness of the quartzite is counteracted by the inclusion with it of the phosphate ore which particles surround and adhere to the silica particles so that wear in the briquette roll pockets is substantially reduced.

More specifically, in our process, finely divided phosphate ore, after being washed free of clay and a fraction of it subjected to froth flotation in granular form, containing quartzite sand with other minor impurities occurring in the materials, is heated in a fluid bed, or other kiln type heating device, to a temperature in the range of 1600 F. to 1900 F. and briquetted. By finely divided phosphate mineral we mean a material of a screen analysis from approximately that which passes a 35 mesh screen to that which is retained on a mesh screen.

3 The quartzite sand is also about 35 mesh to +150 mesh in screen analysis. Under these conditions and with such a mixture, the fine phosphate particles, particularly the finer portions of the screenings become attached to and surround the quartzite sand particles to form an agglomerated coating. The material thus, coming through the heating device is fed hot to the briquette rolls where it is compacted into briquettes.

By operation in this manner we have found that a novel phosphate briquette is produced in that the phosphate particles which adhere to the quartzite sand by becoming plastic in the press at the temperature range indicated form a cementing matrix which holds the separate particles of sand in close relationship to phosphate for reaction in the electric furnace and generally holds all of the phosphate together in a matrix among the particles of quartz. In view of the fact that there is no quartzto-quartz particle contact, a briquette as strong as natural phosphate ore is formed.

The further advantage accruing from this novel form of briquette is that this physically protective coating of phosphate on the quartzite particles greatly reduces the abrasiveness of the mixture in the briquetting apparatus and thereby substantially reduces the rate of wear in the briquette roll pockets.

Hence, operating in accordance with the process of the invention it is possible to start with phosphate ore of relatively low grade, wash it free of accompanying clay slime, subject a portion of it to flotation to develop a phosphate concentrate and then mix portions of concentrate and washed ore, thereby to prepare a phosphate feed for an electric furnace reduction. Formulated with washed ore containing silica sand the furnace feed is ideal from the chemical standpoint; it is a furnace feed with a balanced phosphorus pentoXide, calcium oxide, and silicon dioxide content which, of course, represents a very economical utilization of the constituents of the ore as mined.

The equations governing the electric furnace reduction reaction are essentially as follows:

The calcium silicate slag is tapped from the furnace as liquid; the CO is used as a fuel in the kilns; the P vapor is condensed and recoveerd.

Reference to the drawing will illustrate the nature of the new briquette which is produced by the process of the invention;

FIG. 1 is a sketch of a typical phosphate briquette, substantially of full conventional size; and

FIG. 2 shows a cross-section, enlarged, of the briquette to indicate the nature of the matrix and the cement.

In the section of FIG. 2, sand particles 10 are indicated in somewhat exaggerated size and the phosphate matrix is indicated by the numeral 11.

In view of the fact that the apparatus used for washing the ore, subjecting it to concentration, drying, the fluid bed heating, or the kiln drying and heating, and the actual briquetting machine, are of conventional structure, they are not illustrated in the drawings. A useful process and apparatus for preparing calcined phosphate aggregates is shown'in US. Pat. 3,076,248, Feb. 5, 1963, Darrow et al. and it is to be considered fully incorporated herein.

In testing the operation, the following example indicates the level of control employed over the variables:

EXAMPLE Inherent in the development of briquettes from a North Carolina or Florida granular phosphate ore for electric furnace application is the fact that a blend of an approximate range of 2:1 to 5:1, by weight, of washed ore to concentrate shows the correct range of proportions of phosphorus pentoxide, calcium oxide and silicon dioxide for furnace feed without supplemental silicon dioxide addition. However, the calcining and agglomeration of the fine granular material for furnace feeds presents a problem. Nodulizing in a rotary kiln is not an efiicient or satisfactory means for agglomerating these materials. Grinding and pelletizing with binders added and then subsequent calcination is costly. Hence, the hot briquetting process described herein has advantages derived from the balanced feed of this briquette and the conversion of the finely divided starting feed composition to an appropriate physical form for use in electric furnace operations.

Thus, for hot briquetting purposes, a blend of North Carolina concentrate and washed ore was heated to approximately 1400 F. and briquetted in a 130 ton bri quette press. Pressure sufiicient to induce formation of a phosphate cement in the hot mixture is all that is needed. Excellent briquettes measuring 2 /2 inches X 1 /2 inch X 1 inch were produced from the blend. Size of briquette is not really limited to these dimensions. Ample latitude is possible. This size should be treated as about an useful one for electric furnace feed in that the size of the lumps reduces dusting and favors quick completion of reaction. Too large briquettes give poor reaction in the furnace through creation of heat transfer problems; too small, i.e. pellets, smother gas in the furnace and reduce heat exchange. For production operations, all that is necessary is to establish a continuous fluid bed calciner to heat feed material to briquetting temperatures, form the mix, and feed it from the calciner to a briquetting machine, press and form briquettes.

Specifically in a small scale operation, with the use of a 42 inch fluid bed calciner, a 20 inch diameter, ton press with roll segments, carrying a single roll of pockets, it is possible to briquette a feed quite satisfactorily. Tests conducted with roll segments having pillow pockets measuring 2 /2 X 1 /2 X 1 gave satisfactory briquettes over a wide range of operating temperatures, namely from about 1600 F. to 1900 F. The temperature limit is more a limitation of the materials of construction of the briquetting machine than a limitation of the feed materials themselves. The briquette roll pressure ranging from 28 to 80 tons total pressure on a roll is an adequate operating range. On this basis it is impossible to state the pressure on the individual briquette, because of the nature of the form of the roll and how the pressure would be distributed through the pockets forming the briquettes. The feed compositions suitable for production vary from 0.75 to 1.00 SiO /CaO ratio. Adjustment is made by blending washed ore and flotation concentrate in accordance with the respective analyses needed for the feed.

Samples of briquettes formed under such conditions were tested for their physical strength using a tumbling procedure. This tumble test is an empirical one and it is simply one wherein a number of briquettes, e.g., five briquettes, are charged to a 2 quart jar mill having lifters in it, the mill being turned at a rate of 22 revolutions per minute for a period of 5 minutes. The contents of the mill are then screened and the amount of -6 mesh material and that which is larger is measured as the strength of the briquettes. This is a semi quantitative control test. Actually the measure of required strength of briquette reduces simply to that strength which will give it suflicient resistance to fracture and abrasion in the handling and feeding equipment to give an appropriate furnace feed. In any given plant operation an empirical control test can be devised.

Large scale commercial operation can involve use of a 20 foot calciner and a corresponding large briquette press with several rows of pockets. The pressure per briquette is undeterminable, but is, of course, related to the total pressure on the roll distributed over the number of pockets.

Thus, as a basic generalization for handling of the process, it may be said that when operating with a blend of flotation concentrate and washed ore containing silica sand temperatures as low as about 1600 F., in a pilot briquetting machine with a single row of pockets with pressure as low as about 28 tons, may be used to produce satisfactory briquettes. It is preferable to operate at higher pressure near 50 tons and at a higher temperature preferably of the order of 1600 F. to 1900" F. and generally near the upper limit of about 1900 R, which is a limit imposed by the properties of the materials of construction of the apparatus, not by the properties of the materials being processed. Experimental data indicate that operating temperatures for briquetting up to and well into the plastic range for molding the calcium phosphate ore would be suitable. The only limitation really is to have the ore in a condition such that it is plastic and easily molded or formed into an agglomerate so that cohesion of ore particles is attained, yet not so plastic that it will adhere easily to the roll pockets in the briquetting machine.

A complete analysis of feed components typical of what is used is as follows:

CHEMICAL ANALYSES OF FEEDr-PERCENT WEIGHT [Dry basis] Washed ore Concentrate Total 99. 12 97.

TYPICAL TEST BRIQUETTING CONDITIONS Feed Bed tempertemper- Etiquette ature, ature, weight, F F. grams The tests clearly indicated on the basis of observation, of operation of the briquetting that the optimum conditions for a specific sized pocket, pressure and temperature and particular feed composition utilized in the tests are achieved with:

(l) concentrate and washed ore mixes in the range 1:2 to 1:5, as called for by the analysis;

(2) briquetting temperature of 1400 F. to 1900 F., with operation preferred near the upper level;

(3) briquette weight of about grams or more is preferred when using the 2 /2 x 1 /2 x 1 inch pocket rolls.

With these variables controlled about as indicated we have found briquettes come through with very good tumble strength and little sticking to the machinery. Production rate is a function of the size and speed of the machine.

What is claimed is:

1. A process for briquetting an intimate mixture of a washed granular calcium phosphate ore of 35 to mesh size containing high silica sand of like mesh size and a phosphate ore concentrate of enriched phosphate content of like mesh size, comprising heating the mixture to a temperature in the range from about 1400 F. to 1900 F., thus calcining it, and while the mixture is thus heated, compressing the mixture under high pressure to form briquettes, said briquettes having an internal structure characterized by a matrix of quartz sand substantially uniformly distributed throughout, held together by essentially fused phosphate ore, said briquettes having at least sufficient strength to permit handling with mechanical equipment for feed to an electric furnace, the pro portions of phosphate ore and silica being approximately the stoichiometric levels needed to bring the calcium into calcium silicate during reduction of the calcium phosphate in said ore with carbon in an electric furnace.

2. A process in accordance with claim 1, wherein the washed ore has a P 0 content of about 15% to 20%, a CaO content of about 22% to 30%, and a SiO content of about 40% to 50% and a portion of said ore is concentrate of increased P 0 content and reduced SiO content, and, thereafter, said concentrate is blended with washed ore in a ratio suflicient to bring the SiO content into an approximate balance with the CaO content of the blend, heating the mixture and forming briquettes therefrom.

3. The method in accordance with claim 2, wherein said enriched concentrate and washed ore are blended in a ratio from about 2 parts washed ore to about 1 part concentrate, by weight, to about 5 parts washed ore to about 1 part concentrate, by weight.

References Cited UNITED STATES PATENTS 2,029,309 2/1936 Curtis et al. 23-108 2,776,828 1/ 1957 Marcellus et al. 23-313 3,032,408 5/1962 Baumann 23-313 3,076,248 2/1963 Darrow et al. 264-56 3,189,433 -6/ 1965 Hollingsworth et al. 23-313 3,324,221 6/1967 Hulse et al. 264-332 3,387,923 6/1968 Shen 23-313 FOREIGN PATENTS 1,064,196 4/1967 Great Britain.

481,706 3/ 1952 Canada.

DONALD J. ARNOLD, Primary Examiner I. H. MILLER, Assistant Examiner US. Cl. XJR. 

